Other
Scientific paper
Jul 1978
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1978gecoa..42..945h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 42, Issue 7, pp.945-957
Other
48
Scientific paper
One petrogenetic grid for plagioclase-, spinel- and garnet-lherzolite analogues in the system CaO-MgO-Al 2 O 3 -SiO 2 is presented from 1 bar to 30 kbar and 400 to 1500°C. Another grid for olivine-gabbro, spinel-gabbro and garnet-pyroxenite analogues in the same system is presented from 1 bar to 25 kbar and 500 to 1500°C. Both grids show the distribution of the mineral assemblages and the variations in the composition of clinopyroxene with temperature and pressure. They were developed by applying simple thermodynamic mixing models of clinopyroxene to experimentally determined clino-pyroxene compositions. Calcium tschermak's pyroxene (CaAl 2 SiO 6 ) in complex CaMgSi 2 O 6 -CaAl 2 SiO 6 -Mg 2 Si 2 O 6 clinopyroxenes is best represented by a local charge balance mixing model where a CaAl 2 SiO 6 ( X Ca M 2 )( X Al M 1 ) Enthalpy and entropy changes of subsolidus reactions involving variations in the CaAl 2 SiO 6 and Mg 2 Si 2 O 6 content of clinopyroxene are interdependent due to nonideal mixing of these two end-members. CaAl 2 SiO 6 can strongly reduce the mutual solubility of clinopyroxene and orthopyroxene at moderate pressures and high temperatures. Failure to take this into account can result in temperature underestimates (up to 150°C) of spinel-lherzolites, garnet-pyroxenites, low pressure garnet-lherzolites, spinel-gabbros, and high pressure plagioclase-lherzolites and olivine-gabbros. However, at temperatures and pressures where the Al 2 O 3 content of clinopyroxene is low (e.g. garnet-lherzolite nodules in kimberlite), the mutual solubility is adequantely represented by experimental results in the system CaO-MgO-SiO 2 .
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